NO319083B1 - Crosslinkable polyolefin mixtures, and process for their preparation and use - Google Patents

Description

This invention relates to polyolefins. In one aspect, the invention relates to crosslinkable polyolefin blends, especially polyethylene blends, which have both good processability and fast curing rates. In another aspect, the invention relates to crosslinkable, bidisperse polyolefin mixtures in the same family, with different number average molecular weights, which are suitable for many different manufacturing technologies, for example metal wire and cable extrusion, rotational molding, profile extrusion, injection molding, extrusion blow molding, injection blow molding, thermoforming, drop molding ( "top forming"), pressure blowing, slot die extrusion, plate die extrusion, blown film extrusion and powder coating. In yet another aspect, the invention relates to a method for improving the processability of a polyolefin blend while maintaining a relatively fast curing rate. In yet another aspect, the invention relates to various articles of manufacture, for example metal wire and cable coverings, sealing strips and seals, gaskets, articles made of foamed and/or sponge-like cross-linked polyolefin with either closed or open cell structure and/or combinations of these, hollow containers , hollow tanks, medical instruments, drapes and covers, healthcare and personal care articles, fibers, ribbons, hoses, tubes and stockings, bellows, boots, gaiters, footwear, made from crosslinkable polyolefin blends.

Polyolefin blends can be processed into shaped articles using various techniques. For many end uses, the articles should be cross-linked. Cross-linking usually takes place during or after the processing step.

Such polyolefin mixtures should generally be easily processed and should be cross-linked or cured relatively quickly. The rate of cure, that is the time required to achieve a desired or optimum state of cure, is generally considered to be dependent on a number of factors.

It is generally accepted that the higher the curing temperature, or the greater the molecular weight, the higher the curing speed.

Short curing times are desirable, as they increase productivity and reduce work-in-progress inventory. On the other hand, less stringent (for example, lower temperature) curing conditions are desirable, for example to reduce energy consumption or to make the handling of manufactured and semi-finished parts easier.

Another important aspect is the workability of the mixture. Various characteristics have been proposed as measures of the processability of a polymer mixture. Among the best known is the Melt Index, also called l2, measured according to ASTM D-1238 (Condition 190/2.16, formerly Condition E). The melt index is a suitable processed bareness indicator when comparing polymers within the same family. In general, the lower the melt index value, the more difficult it is to process the polymer. Higher molecular weight polyolefins usually have a lower melt index, and higher molecular weight polyolefins are therefore more difficult to process. During injection molding, for example, low values of the melt index result in higher pressure requirements for filling the mold cavity, and in the case of exceptionally low melt index values, it may be impossible to fill the mold cavity completely, in addition to consuming unnecessary amounts of energy to make the polymer flow.

Another measure of processability is the shear rate (or shear stress) at which the polyolefin blend exhibits melt fracture during processing. It is preferred that the shear rate (or shear stress) at which onset of melt fracture takes place is as high as possible to enable high fabrication rates for finished and semi-finished articles. It is generally believed that for a given molecular weight distribution, the shear rate corresponding to the onset of surface melt fracture decreases as the average molecular weight of a polyolefin blend is increased.

John Dealy describes in "Melt Rheology and Its Role in Plastics Processing", published by Van Nostrand Reinhold Co. in 1990, on page 597 that the measurement of the melt index according to ASTM D-1238, where different loads are used, will give an estimate of the dependence of the shear rate on the melt viscosity, which is sensitive to the weight average molecular weight (M„) and the number average molecular weight (Mn) . Most often, the ratio between the melting index at 190°C measured at less than 10 kg, in relation to the melting index measured at less than 2.16 kg, is used as a measure of the sensitivity to shear thinning. This relationship is usually referred to as

Stehling et al. teaches in US patents no. 5,387,630 and 5,382,631 (corresponding to WO 90/03414) that specific mixtures of linear low-density polyethylenes with a narrow molecular weight distribution, which are mainly characterized by a polydispersity index ( MJMJ in the mixture exceeding a value pd 3, provided that the value of MJMn for each of the mixture components is less than 3, provides increased tear resistance and reduced "extractability" compared to prior art mixtures. This publication does not teach cross-linking of these mixtures.

A polyolefin blend that has both a high cure rate and excellent processability has been identified by the industry as a desirable product, and the conflicting demands of low molecular weight (for improving processability) and high molecular weight (for increasing cure rate) have made great efforts by professionals in the field to try to find a balance between the two conflicting requirements, or to optimize the processing machinery and operating conditions to minimize the disadvantages, or to try to discover alternative methods to overcome the problem.

Processors of crosslinkable polyolefin mixtures use great effort to, for example, select accurate tool dimensions to minimize the degree of pressure drop as polymer is forced to flow through the forming nozzles, whereby the use of polyolefins with higher molecular weight becomes possible, and shorter curing times can be used, something that is beneficial. However, optimal molding tool and die design for one polyolefin blend is not necessarily beneficial for another polyolefin blend, and it therefore becomes necessary to change the flow path and molding die design with each change in production, and in any case the benefits achieved are small if the initial design corresponds to good industrial practice.

Various forms of "processing aid" have been proposed. Such processing aids include, for example, calcium stearate, zinc stearate, magnesium stearate, mineral oils of different viscosity (typically from 50 to 150 centistokes), microcrystalline paraffin wax and polyethylene glycols, either alone or in combination. Varrall et al. teaches in WO 91/08262 the addition of up to 10% polyethylene wax to LLDPE for the purpose of improving the workability of silane-crosslinkable LLDPE intended for the production of metal wire and cable insulation and coverings. The purpose is usually to reduce the friction between the polymer melt and the inner surfaces of the forming die and associated flow passages. The additive typically migrates at least partially from the molten polymer mixture to the interface with the nozzle, whereby this interface is lubricated. It is a characteristic of such lubricants that they corrode away from the polymer/metal interface during use, and they must therefore be regularly supplemented by constant addition of the active process aid additive. This significantly increases the costs of the processing operation. Since it is not uncommon for the corroded processing aid to remain in the bulk or attached to the surface of the processed polymer, this causes a lack of purity that represents potential problems in the use and durability of the manufactured article, such as poor sealing properties, greater difficulty in to press on the surface, as well as better electrical properties.

A method for improving the processability of polyethylenes with a narrow molecular weight distribution, such as LLDPE, is proposed in WO 91/08262. This patent proposes the use of blends of LDPE with LLDPE for silane-crosslinkable polyethylene blends intended for metal wire and cable insulation and cladding, for improving the extrudability of LLDPE blends.

An alternative method has been to use mixtures of high molecular weight polyethylene with low molecular weight polyethylene. For example, it is mentioned in WO 91/08262 that a mixture of a first LLDPE with a melt index of from 0.2 to 10, and a second LLDPE with a melt index of 20-50, most preferably in the proportions of 30-70% of the first component and 70-30% of the second component, will provide a good balance between curing speed and workability for silane cross-linking systems for the production of metal wire and cable coating and insulation. However, this patent does not provide any example of such a mixture.

Yet another example of an attempt to define optimum mixing conditions and composition is given in EP 584 927, which teaches that the addition of a small amount of an "auxiliary" polymer component which can be co-crystallized with a "main" polymer component which from ethylene and possibly at least one higher alpha-olefin, will reduce the time required to achieve optimal curing without the workability deteriorating to any significant extent. However, it also teaches that it is preferred to add a polymer processing aid in the form of a fluorinated polymer and/or a polymer derived from ethylene and optionally at least one olefinically unsaturated co-monomer with a melt index of at least 5 grams per 10 minutes greater than for the "main" component.

Despite the previous attempts, there is a desire to provide crosslinkable polyethylene blends which have both good workability and good curing speed.

The present invention provides compositions with an improved combination of workability and increased cure rate. The polyolefin mixtures according to the invention comprise two or more olefin polymer components, preferably ethylene polymer, which satisfy the following conditions: where A„ A2, Mn1 and M^ are derived from the molecular weight distribution of the mixture obtained by means of gel permeation chromatography (GPC) by derivation (de-convoluting ») of the relative response (RR) as a function of molecular weight (MW) while fitting RR and MW to the following function, which is a weighted sum of two log-normal distribution functions:

using a non-linear regression technique to obtain values for A„ A2, m, Mi. cf, and o2, where

MW is the molecular weight value for GPC,

RR is the relative response that for an individual RR/MW data set is

RR[i] = Normalized height [i]/(log (MW[i-1]) - log (MW[i]))

where the normalized height [i] is the GPC yield for the corresponding MW(i);

\Lf and a,2 represent the mean and standard deviation of the first log-normal distribution; \ i2 and a* represent the mean and standard deviation of the second log-normal distribution;

and polyolefin blends! has a total density of less than 0.907 g/cm<3>.

The present invention further provides a method for producing a polyolefin mixture, which comprises: a) producing a first olefin polymer and a second olefin polymer; b) mixing the first and second olefin polymer such that the first and second olefin polymer are mixed homogeneously, providing a polyolefin mixture satisfying the following conditions: where Mn1, M^, A, and A2, are obtained by the molecular weight distribution of the mixture obtained by means of gel permeation chromatography (GPC) by deriving the relative response (RR) as a function of the molecular weight (MW) to fit the RR and MW to the following function, which is a weighted sum of two log-normal distribution functions:

using a non-linear regression technique to obtain values for A,, A,, m, h>, er, and aj, where

MW is the molecular weight value for GPC,

RR is the relative response that for an individual RR/Mw data set is

RR[t] = Normalized height [i]/(log (MW [i-1]) - log (MW [i]))

where the normalized height [i] is the GPC yield for the corresponding MW (i);

H, and c,<2> represent the mean and standard deviation of the first log-normal distribution;

m and c2<2> represent the mean and standard deviation of the second log-normal distribution;

and the polyolefin mixture has a total density of less than 0.907 g/cm<3>.

Another aspect of the present invention is a use of a cross-linked polyolefin composition obtained by curing the composition according to any one of claims 11 to 15 for the production of an article selected from the group comprising, films, extrusion coated and laminated foils, extruded and calendered sheets and foils, metal wire and cable coverings, sealing strips and seals, gaskets, articles made of foamed and/or sponge-like cross-linked polyolefin with either closed or open cell structure and/or combinations thereof, hollow containers, hollow tanks, medical instruments, molded cups , drapes and covers, health care and personal care articles, fibers, ribbons, hoses, tubes and stockings, bellows, boots, gaiters, footwear, shoe soles and uppers. Fig. 1 illustrates a gel permeation chromatogram for the polyethylene Tafmer™ P0480 according to the state of the art. Fig. 2 illustrates a gel permeation chromatogram for a prior art polyethylene with a total Mn of 19,400, a total Mw of 64,100 and a polydispersity index of 3.3. Fig. 3 illustrates a gel permeation chromatogram for a pattern olefin mixture according to the present invention, corresponding to Example 2, with a total Mn of 29,900, a total Mw of 124,000 and a polydispersity index of 4.14. The bi-disperse nature of the molecular weight distribution is clearly evident. Fig. 4 illustrates a gel permeation chromatogram for another pattern olefin mixture according to this invention, corresponding to Example 3, with a total Mn of 6950, a total Mw of 85,000 and a polydispersity index of 12.2. Fig. 4 represents an extreme case of bi-dispersity. Fig. 5 represents the shear rate at which onset of surface melt fracture takes place as a function of total M„ for the polymer mixtures according to Table IV. Fig. 6 shows melt viscosity as a function of total Mn for the mixtures according to table V. Fig. 7 shows the variation in curing time for the series of peroxide-crosslinkable mixtures according to table VI as a function of total Mn. Fig. 8 shows the variation in time to 175% cure of thermosetting target material for the series of air-cured silane crosslinkable compounds listed in Table VII, as a function of total Mn. Fig. 9 shows the effect of Mn on the time to 175% cure of thermosetting target material for a series of silane crosslinkable polyethylene blends cured in water at 60°C as shown in Table VIII. The data points representing the present invention are highlighted by means of arrows. Fig. 10 graphically illustrates the difference between the mixtures according to the state of the art and the present invention, based on the discharge parameters Mn1, MnZ, A, and Aj shown in Table IX.

Where the designations M„ and Mw or the designations total Mn and total M„ are used in the patent application, these indicate respectively the number-average molecular weight and the weight-average molecular weight for the total polyolefin mixture, in contrast to Mn1 and Mn2, which represent properties of the polyolefin mixture derived by the specified derivation method.

According to the present invention, it has surprisingly been shown that the mixtures according to the invention exhibit an improved combination of workability and curing speed compared to mixtures according to the state of the art with a similar total number average molecular weight, M„, and with a similar density. The present inventors have found that such improved properties are related to specific molecular weight parameters as explained in more detail below.

The parameters A,, Aj, Mn1 and Mn2 used to describe the present polyolefin mixtures are obtained by GPC analysis of samples of these mixtures.

Fig. 1 and fig. 2 are representative GPC curves for polyolefin blends according to the state of the art. Fig. 3 and fig. 4 are representative GPC curves for polyolefin blends according to the present invention.

The samples are analyzed by gel permeation chromatography on a Waters 150°C high temperature chromatography unit equipped with three pI-gel 10 nm mixed percent columns operated at a system temperature of 140°C. The solvent is 1,2,4-trichlorobenzene, from which 0.2% by weight solutions of the samples are prepared for injection. The flow rate is 1.0 milliliters per minute, and the injection size is 200 microliters.

The molecular weight determination is derived by using polystyrene standards with a narrow molecular weight distribution (from Polymer Laboratories) together with the elution volumes for these. The equivalent polyethylene molecular weights are determined using appropriate Mark-Houwink coefficients for polyethylene and polystyrene (as described by Williams and Ward in Journal of Polymer Science, Polymer Letters, vol. 6, (621) 1968), obtaining of the following equation:

In this equation, a = 0.4316 and b = 1.0. Weight average molecular weight, M,,,, is calculated in the usual way according to the following formula: Mw = 2 w<*> M,, where w. and M. t are respectively the weight fraction and the molecular weight of the i<end> fraction eluted from the GPC column.

By subjecting the GPC curves to derivation by non-linear regression based on the double log-normal distribution function stated above, the values for A,, A2, a,, a2, n, and m can be calculated for each gel permeation chromatogram. The values for u., and of m and a2, respectively, are used to calculate the values for Mn1 and Mn2.

The above derivation method provides two log-normal distributions, each of which is characterized by the values of Mni, a, and n,. The advantages of the present invention were found to exist when Mn1 and Mn2 satisfy the ratio specified above. Mn1 can be considered to represent the number average molecular weight for the log-normal distribution corresponding to the fractions with higher molecular weight, obtained by the specified derivation method. Analogously, Mn2 can be considered to represent the number average molecular weight for the log-normal distribution corresponding to the fractions with lower molecular weight, calculated by the specified method.

The greater the ratio of Mn1 to Mn2, the greater the processability of the polyolefin blend. However, to maintain the desired cure rate for the mixture, the value of Mn1/Mn2 must be greater than 5, preferably greater than 6.5, and more preferably greater than 7.0. If the ratio MJM^ is less than 5, the benefit of the cure rate improvement will not be obtained.

According to the present invention, it is preferred that the mixture comprises a mixture of components selected from the families of homogeneous linear PE or from the family of mainly linear ethylene polymers. Most preferred are mixtures obtained from two SLEPs.

The value Mn2 is greater than 3,000 and is preferably greater than 4,000.

Furthermore, the ratio A,/(A, + AJ, which can be regarded as the relative contribution of the first log-normal distribution (which is characterized by ji, and a,) to the weighted sum of the first and second log-normal distributions obtained by application of the double log-normal derivative function described above, for the gel permeation chromatogram of the mixture, lie within certain limits. The ratio of A, to (A, + Aj) must be greater than or equal to 0.15, and preferably greater than or equal to 0 .2, and more preferably greater than or equal to 0.25. The ratio between A, and (A, + Aj) must also be less than or equal to 0.7, preferably less than or equal to 0.65,

and more preferably less than or equal to 0.6. If the ratio of A, and (A, + A2) is not in the range of 0.7 to 0.15, the cure rate will be lower than desired and the shear-thinning effect associated with bi-modal molecular weight distribution polymers will be reduced, which results in a deterioration of machinability. ■

The values for A 1 , A 2 , Mn 1 and Mn 2 are obtained using non-linear least squares analysis of a series of data points produced by GPC expressed in terms of log MW plotted against relative response. The real increments relative

response, (Relative response [i]), corresponding to a particular log MW[i], is expressed by the formula:

where normalized height (i) and MW [i-l] and MW [i] are obtained from the GPC data.

Because in equation 1, the relative response is zero. The sum of all the normalized heights [i] is equal to 1.

The numerical method used for curve fitting is the method for Choleski decomposition with numerically determined first derivatives with errors in all variables as described in Technical University of Eindhoven (The Netherlands) PP-5. 3, Nonfinear Regression Without Limits, 1989 TUE-RC 68438 and in "An Introduction to Numerical Analysis" by K.E. Atkinson, published by John Wiley & Sons, Inc. in 1978. This numerical analysis can be performed using commercially available computer programs such as RRGraph™ from the Reactor Research Foundation, which is registered with the Chamber of Commerce, Delft, The Netherlands, under registration no. S145980. Such nonlinear least squares analysis uses the double log-normal distribution function for the series of data points represented by log (MW [i]) plotted against relative response [i] to determine the values of A,, A2, a,, a2, m and n2 which provides the best fit to the data points for log (MW [i]) compared with relative response [i]. Initial output values for the parameters A,, A2, (equal to 1 - A,), \iv y^, are, and o2 are based on knowledge of the composition and/or shape of the GPC curve. Starting values which have been found to give successful discharges are as follows: A, = 0.4-0.6; n, approx. 5; n2 approx. 4; o, and a2 each approx. 0.35. If the derivation is not successful, the output values can be adjusted based on the shape of the GPC curve.

The values A1, A2, Mn1 and Mn2 calculated as explained above are obtained from the mixture and, in the case of a real mixture, may not correspond to the respective actual mixing ratios and number average molecular weights of the mixture components. The ratio between A, and (A, + Aj) thus does not necessarily correspond to the actual mixing conditions in the case where the mixture is made by mixing two or more polymer components. Nor do the values for Mn1 and Mn2 necessarily correspond to the real values for M„ which would have been obtained by gel permeation chromatography of the separate mixture components before mixing. The present mixtures may comprise more than two olefin polymer components, particularly when they are prepared by means of mixing, provided that the total mixture satisfies the conditions specified above.

The polyolefin mixtures according to this invention can generally be obtained by mixing two or more polyolefins, preferably selected from the same family. As such, polyolefin blends of this invention include blends of two or more linear PEs or blends of two or more LDPEs, or of two or more SLEPs, but not blends of linear PE and LDPE.

Low-density polyethylene ("LDPE") is usually produced at high pressure using free-radical initiators, and typically has a density in the range from 915 to 940 kilograms per cubic meters (kg/m<3>). LDPE is also known as "branched" polyethylene due to the relatively large number of long-chain branches that emanate from the main polymer skeleton. LDPE comprises ethylene and may optionally comprise small amounts (for example up to approx. 5% by weight) of comonomers such as propylene, butene-1, vinyl acetate and butyl acrylate.

Ethylene polymers and copolymers prepared using a coordination catalyst such as a Ziegler or Phillips catalyst are commonly known as linear polyethylene ("linear PED") due to the substantial absence of protruding branched chains of polymerized monomer units. from the skeleton. Linear PE includes HPDE and LLDPE, the latter designation for the present forms including ULDPE and VLDPE. High-density polyethylene ("HDPE"), which typically has a density of 941-967 kg/m<3>, is typically a linear homopolymer of ethylene or an interpolymer of ethylene and a small amount of α-olefin, and it contains relatively few branched chains in relation to the various linear interpolymers of ethylene and an α-olefin. As used herein, "interpolymer" denotes a polymer of two or more comonomers, for example a copolymer or terpolymer.

Linear low-density polyethylene ("LLDPE") is typically an interpolymer of ethylene and an α-olefin with 3-12 carbon atoms, preferably 4-8 carbon atoms (for example 1-butene or 1-octene) which has a sufficient content of α-olefins to reduce the density of the interpolymer to that of LDPE. LLDPE is a member of the family of linear PEs. When the interpolymer contains even more α-olefin, the density will fall below approx. 910 kg/m<3>, and these interpolymers are known as ultra-low density polyethylene ("ULDPE") or very low density polyethylene ("VLDPE"). The densities of these linear polymers are usually in the range of 865 to 910 kg/m<3>. VLDPE and ULDPE are both members of the family of linear PEs.

Linear ethylene interpolymers include homogeneously branched and heterogeneously branched LLDPEs. Heterogeneous LLDPEs usually have a very wide, non-uniform distribution of comonomer content, that is to say that some polymer molecules have a relatively high α-olefin comonomer content, while others have a relatively low content. Generally, the polymer molecules with a low comonomer content are relatively more crystalline and have a high melting temperature, while the polymer molecules with a high comonomer content are more amorphous and melt at a lower temperature.

Homogeneously branched linear polyethylenes that can be used in the practice of this invention (also called "homogeneous linear polyethylene" or "homogeneous linear PE" or "homogeneous LLDPE") are known, and their production method is described in US patent 3,645,992. Examples of homogeneous LLDPEs are polymers of the type Tafmer™ (trademark belonging to Mitsui) and Exact™ (trademark belonging to Exxon).

Another family of olefin polymers are the predominantly linear olefin polymers. The substantially linear olefin polymers, and more particularly substantially linear ethylene polymers (SLEPs) and the process for their preparation are described in US Patent Nos. 5,272,236, 5,278,272 and 5,380,810. SLEPs are supplied by DuPont Dow Elastomers L.L.C as polyolefin elastomers of the Engage® type, and by The Dow Chemical Company as polyolefin plastomers of the Affinity® type.

As used herein, "mainly linear" means that the polymer mass has on average from 0.01 long-chain branches per 1000 carbon atoms to 3 long-chain branches per 1000 carbon atoms, preferably from 0.01 long-chain branches per 1000 carbon atoms to 1 long-chain branching per 1000 carbon atoms, and more preferably from 0.05 long-chain branches per 1000 carbon atoms to 1 long-chain branching per 1000 carbon atoms. In contrast, "linear" simply indicates that the polymer backbone is substituted with less than 0.01 long-chain branches per 1000 carbon atoms.

The term "mass polymer" used herein denotes the polymer obtained by the polymerization process, and in the case of the mainly linear polymers, it includes molecules without long chain branching as well as molecules with long chain branching. A "mass polymer" thus includes all molecules formed during polymerization. It must be clear that in the case of the predominantly linear polymers, not all molecules have long-chain branching, but a sufficient amount do so that the average content of long-chain branching for the bulk polymer positively affects the melt rheology (that is, the melt fracture properties).

Long chain branching (LCB) is defined herein as a chain length that is at least 1 carbon atom less than the number of carbon atoms in the comonomer, while short chain branching (SCB) is defined herein as a chain length with the same number of carbon atoms in the comonomer residue after the is incorporated into the polymer molecular skeleton. For example, a predominantly linear ethylene/1-octene polymer has skeletons with long-chain branches with a length of at least 7 carbon atoms, but it also has short-chain branches with a length of only 6 carbon atoms.

Long-chain branching can be distinguished from short-chain branching using nuclear magnetic <13>C resonance (MNR) spectroscopy and to a limited extent, for example for ethylene homopolymers, it can be quantified using the method according to Randall (Rev. Macromol . Chem. Phys., C29 (2&3), pp. 285-297). However, for practical reasons, current <13>C-NMR spectroscopy cannot determine the length of a long-chain branch beyond ca. 6 carbon atoms, and as such this analytical technique cannot distinguish between a 7-carbon chain and a 70-carbon chain. The long-chain branch may be as long as, or approximately the same length as, the length of the polymer backbone.

US patent 4,500,648 teaches that the frequency of long chain branching (LCB) can be represented by the equation LCB = b/Mw, where b is the weight average number of long chain branches per molecule, and Mw is the weight average molecular weight. The molecular weight averages and character properties of the long chain branches are determined by gel permeation chromatography and intrinsic viscosity methods.

As used herein, "homogeneously branched" indicates that the comonomer is randomly distributed in a given molecule, and that substantially all of the copolymer molecules have the same ethylene/comonomer ratio. The distribution or homogeneity of comonomer branching for the mainly linear ethylene interpolymers and homopolymers is characterized by its SCBDI (Short Chain Branching Distribution Index) or CDBI (Composition Distribution Branching Index), and it is defined as the percentage by weight of the polymer molecules that have a comonomer content within 50% of the total molar median comonomer content. The CDBI of a polymer is readily calculated from data obtained by techniques known in the art, such as, for example, temperature rise-elution fractionation (abbreviated herein as "TREF"), as described, for example, by Wild et al., Journal of Polymer Science , Poly. Phys. Ed., vol. 20, p. 441 (1982) or in US Patent 4,798,081. The SCBDI or CDBI of the substantially linear homogeneously branched interpolymers in the preferred polymer blends of the present invention is generally greater than 30%, preferably greater than 50%, and especially greater than 80%.

Both the linear homogeneously branched polymers and the SLEPs used in this invention have a single melting peak, as measured using differential scanning calorimetry (DSC), in contrast to heterogeneously branched linear ethylene polymers, which have two or more melting peaks due to their wide branching distribution.

The unique property of the SLEPs is a very unexpected flow property where the polymer's l10/l2 value is essentially independent of the polymer's polydispersity index (ie MJMn). This is in contrast to common linear homogeneously branched and linear heterogeneously branched polyethylene resins with rheological properties such that to increase the l10/l2 value, the polydispersity index must also be increased. The melt flow ratio, measured as l10/l2 (ASTM D-1238) is preferably greater than or equal to 5.63, and is preferably at least 6.5, more preferably at least 7 and may be as high as 20, preferably up to 15, and more preferably up to 10.

These unique SLEPs are prepared using forced geometry (CGC) catalysts and are characterized by a narrow molecular weight distribution and, if an interpolymer, by a narrow or homogeneous comonomer distribution. The molecular weight distribution {MJMn), measured by gel permeation chromatography (GPC), is preferably defined by the equation:

and is usually less than 5, preferably between 1.5 and 2.5, and especially from 1.7 to 2.3.

The preferred melt index, measured as l2 (ASTM D-1238, condition 190/2.16 (formerly condition E)) is from 0.1 g/10 minutes to 100 g/10 minutes, more preferably 1-20 g/10 minutes . The preferred substantially linear ethylene polymers used in the practice of this invention are typically homogeneously branched and do not have any measurable high density fraction (that is, as measured by temperature rise elution fractionation), for example they do not contain any polymer fraction having any degree of branching of below or equal to 2 methyl groups per 1000 carbon atoms.

Other basic characteristics of these substantially linear ethylene polymers include a low residue content (that is, low concentrations in the substantially linear ethylene polymer of the catalyst used to make the polymer, unreacted comonomers, if any, and low molecular weight oligomers made during of the polymerization) and a regulated molecular structure that provides good workability even though the molecular weight distribution may be narrow compared to ordinary olefin polymers.

The mainly linear olefin polymers used to make the polymer mixtures according to this invention preferably include mainly linear ethylene polymers, both homo- and interpolymers. The mainly linear ethylene polymers preferably comprise between 95 and 50% by weight of ethylene and between 5 and

50% by weight of at least one ct-olefin comonomer, more preferably 10-25% by weight of at least one cc-olefin comonomer. The percentage of comonomers is measured by means of infrared spectroscopy according to ASTM D-2238 method B. These mainly linear ethylene polymers as well as the homogeneous linear polyethylenes are typically copolymers of

ethylene and an α-olefin comonomer with 3-20 carbon atoms (for example propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, 1-heptene, 1-octene and styrene) with a density of from 850 to 967 kg/m<3>, preferably from 865 to 960 kg/m<3>. The comonomer is preferably an α-olefin with 4-10 carbon atoms, more preferably with 5-10 carbon atoms. Particularly preferred are 4-methyl-pentene-1,1-hexene and 1-octene.

In the case of the predominantly linear ethylene polymers, the Is ratios indicate the degree of long-chain branching, that is, the greater the l10/l2 ratio, the more long-chain branching there is in the polymer.

The "rheological processing index" (Pl) is the apparent viscosity (in kpoise) of a polymer measured using a gas extrusion rheometer (GER). The gas extrusion rheometer is described by M. Shida, R.N. Shroff bg LV. Cancio in Polymer Engineering Science, vol. 17, No. 11, p. 770 (1977) and in "Rheometers for Molten Plastics" by John Dealy, published by Van Nostrand Reinhold Co. (1982) at pp. 97-99. GER experiments are carried out at a temperature of 190°C at nitrogen pressure of between 1.7 and 38 MPa overpressure using a 20:1 L/D nozzle with a diameter of approx. 7.54 cm, with an entry angle of 180°. In the case of the preferred substantially linear ethylene polymers described herein for the polymer compositions of the present invention, Pl is the apparent viscosity (in kpoise) of a material as measured by the GER at an apparent shear stress of 2.15 x 10<6> dynes per cm<2>. These substantially linear ethylene interpolymers and homopolymers preferably have a Pl in the range of 0.01 to 50 kpoise, preferably 15 kpoise or lower, which is less than or equal to 70% of the Pl of a comparative linear ethylene polymer (either a Ziegler polymerized polymer or a linear uniformly branched polymer as described in US Patent 3,645,992) at about the same \ 2 and MJMn.

A diagram of apparent shear stress compared with apparent la-. tending speed is used for identification of the melt fracture phenomenon. According to Ramamurthy in Journal of Rheology, 30(2), 337-357, 1986, the observed extrudate irregularities above a certain critical flow rate can be broadly classified into two main types: surface melt fracture and bulk melt fracture.

Surface melt fracture occurs under apparently steady flow conditions and is detailed in the range from loss of specular film gloss to the more severe form of "shark skin". In this description, the onset of surface melt fracture (OSMF) is characterized by the visual appearance of the surface roughness of an extrudate from a capillary rheometer. The critical shear rate at the onset of surface melt fracture for the preferred substantially linear ethylene interpolymers and homopolymers for use in this invention is at least 50% greater than the critical shear rate at the onset of surface melt fracture for a linear comparison ethylene polymer of approximately the same \2 and MJMn .

Mass melt fracture occurs under unsteady extrusion flow conditions and, in detail, ranges from regular (eg, alternating rough and smooth, or helical) to random distortions. For commercial acceptance (for example in blown film products and bags made from them) the surface defects should be minimal, if not absent, for good film quality and properties. The critical shear stress at the onset of mass melt failure for the mainly linear ethylene interpolymers and homopolymers used for the production of the preferred polymer mixtures according to the present invention is greater than approx. 4 x 10<6> dy-ne/cm<2>. The critical shear rate at onset of surface melt fracture (OSMF) and onset of mass melt fracture (OGMF) will be used herein based on the changes in surface roughness and shapes of the extrudates extruded with a GER.

The rheological behavior of predominantly linear ethylene polymers can also be characterized by the Dow Rheology Index ("DRI"), which expresses a polymer's "normalized relaxation time as a result of long-chain branching". (See S. Lai and G.W. Knight "ANTEC '93 Proceedings", INSITE Technology Polyolefins (ITP) - New Rules in the Structure/Rheology Relationship of Ethylene-a-Olefin Copoly-mers", New Orleans, Louisiana, La., May 1993). DRI values range from 0, for polymers that have no measurable long-chain branching (eg, Taf-mer™ products supplied by Mitsui Petrochemical Industries, and Exact™ products supplied by Exxon Chemical Company), to 15, and is independent of the melting index. Generally, in the case of ethylene polymers at low to medium pressure (especially at lower densities), DRI provides improved correlations for melt elasticity and high shear flowability over correlations for the same tested with melt flow conditions. For the substantially linear ethylene polymers useful in this invention, the DRI is preferably at least 0.1, and more preferably at least 0.5, and most preferably at least 0.8. DRI can be calculated from equation 1: where t0 is the characteristic relaxation time for the material and t|0 is zero shear viscosity for the material. Both t0 and r\0 are the "best suited" values determined by a non-linear regression of the experimental data for the cross equation (see equation 2), i.e.

where n is the material's power law index ("power law index") and ti and y are respectively the measured viscosity and shear rate (row sec.'<1>). Baseline determination of viscosity and shear rate data is obtained using a Rheometrics Mechanical Spectrometer (RMS-800) under dynamic sweep mode from 0.1 to 100 radians/second at 190°C and a Gas Extrusion Rheometer (GER) at extrusion

pressures of from 6.89 to 34.5 MPa, corresponding to a shear stress of from 0.086 to 0.43 MPa, using a 20:1 L/D die of 0.754 mm diameter at 190°C. Specific material determinations can be carried out at from 140 to 190°C as needed to allow for melt index variations.

The mixtures can be prepared by physical mixing of two or more identical polyolefins, or by mixing in the reactor. Preparation of the present mixtures by physical mixing includes dry mixing, melt mixing and solution mixing, i.e. dissolving one or both components in a suitable solvent, such as, for example, a hydrocarbon, and mixing the components, followed by removal of the solvent or solvents. Mixing in the reactor typically involves mixing the components in a polymerization reactor, during or after preparation of one or both components. Both types of mixing, i.e. physical mixing and mixing in the reactor, are well known. The present mixtures are preferably prepared by mixing in the reactor using two reactors operated in series or parallel, or by using two or more catalysts in a single reactor or combinations of multiple catalysts and multiple reactors. The general principle for the production of polymer mixtures by mixing in the reactor using two or more catalysts in a single reactor or combinations of multiple catalysts and multiple reactors is described in WO 93/13143, EP-A-619827 and US patent 3 914 342. The present polyolefin mixtures can be produced by choosing the appropriate catalyst and process conditions with a view to the final properties of the mixture.

The polyolefin composition according to this invention can be crosslinked or cured according to any method known for crosslinking saturated polyolefin compositions. Suitable techniques for introducing crosslinks between different molecular chains in a predominantly saturated polymer such as polyethylene include a number of mechanisms such as reaction with the polyethylene by a peroxide or other free radical generator, and optionally a suitable co-agent and/or catalyst and/or activator mixed and/or accelerator and/or promoter such as triallyl cyanurate or elemental sulphur. The reaction is usually initiated by heating the shaped article.

The desired melt index will generally depend on the intended end use of the article produced from the mixture and the method of producing the article from the crosslinkable mixture, and may be from 0.01 to 100 grams per 10 minutes. A melting index value of between 0.2 and 5 grams per For example, 10 minutes is preferred for articles to be produced by extrusion processes such as the blown film technique. A lower melt index value will usually be associated with articles that require higher resistance to neglect, or

ke which is exposed to stress-break resistance requirements due to environmental loads. Articles for production by injection molding will typically be produced from mixtures according to the invention with a melt index of from 4 to 100 grams per 10 minutes,

and most preferably approx. 5-25 grams per 10 minutes. Those skilled in the art will apply the usual rules for determining the most appropriate value for desirable melt index for the intended use of the mixture.

The mixtures according to the present invention preferably have a total density of at least 0.850 g/cm<3>, preferably at least 0.855 g/cm<3>, more preferably at least 0.860 g/cm<3>. The total densities are ■ ■■ below 0.907 g/cm<3>, preferably below 0.900 g/cm<3>, more preferably below 0.890 g/cm<3> and most preferably below 0.885 g/cm<3>. Mixtures with densities below 0.900 g/cm<3> are very suitable for use as coating or insulation for metal wire and cables, especially for flexible

le coatings and insulations.

British Patent 1 286 460 (corresponding to US patent 3 646 155) teaches that chemically reactive compounds can be grafted to the polymer skeleton in such a way that subsequent reactions can take place between the thus grafted compounds linked to different polymer molecular chains so that it is formed crosslinks between these polymer molecular chains. Examples of such a method are the so-called "silane crosslinking process" where unsaturated silanes are grafted onto a polymer, which grafted silanes can then react with moisture in the presence of a catalyst to form crosslinks between polymer chains. Unsaturated silanes suitable for grafting onto a base polymer include silanes of the general formula where R' represents a hydrogen atom or a methyl group; x and y are 0 or 1, with the proviso that when x is 1, y is 1; n is an integer from 1 to 12, preferably 1-4; and each R independently represents a hydrolysable organic group such as an alkoxy group with from 1 to 12 carbon atoms (for example methoxy, ethoxy or butoxy), an aryloxy group (for example phenoxy), aroxy (for example benzyloxy), aliphatic acyloxy with from 1 to 12 carbon atoms (for example formyloxy, acetyloxy or propanoyloxy), oximo or substituted amino groups (alkylamino or arylamino) or a lower alkyl group with from 1 to 6 carbon atoms, with the proviso that no more than one of the three R groups are alkyl. Such silanes can be grafted onto the polyolefin mixtures either before or during a forming or casting operation. The silane can be grafted to the polymer by any conventional method, typically in the presence of a free-radical initiator, for example an organic initiator or ionizing radiation. Organic initiators are preferred, such as organic peroxides, for example dicumyl peroxide, t-butyl perbenzoate, benzoyl peroxide, cumene hydroperoxide, t-butyl peroctoate or methyl ethyl ketone peroxide. The amount of initiator can vary, but it is typically present in an amount of at least 0.04 parts per hundred, bases on the polyolefin mixture (phr), preferably at least 0.06 phr. The amount of initiator typically does not exceed 0.15 phr, and preferably does not exceed approx. 0.10 phr. The ratio between silane and initiator can also vary greatly, but the typical ratio between silane and initiator is between 10:1 and 30:1, preferably between 18:1 and 24:1.

Cross-linking of the silane-grafted mixture is carried out by bringing the silane-grafted mixture into contact with water or another hydrogen-active compound. The water or compound is allowed to enter the polymer from the atmosphere or from a water bath or a "sauna", or by incorporating a substance into the polymer which can release water under suitable conditions, for example heating a polymer containing a hydrated filler such as aluminum trihydro- page The crosslinking reaction usually requires a crosslinking catalyst, which may include a crosslinking agent, activator, promoter and/or accelerator, and combinations thereof. These catalysts usually include organic bases, carboxylic acids and organometallic compounds including organic titanates and complexes or carboxylates of lead, cobalt, iron, nickel, zinc and tin; dibutyltin dilaurate, dioctyltin maleate, dibutyltin diacetate, dibutyltin dioctoate, tin(II) acetate, tin(II) octoate, lead naphthenate, zinc caprylate and cobalt naphthenate. Tin carboxylate, especially dibutyl tin dilaurate and dioctyl tin maleate, are particularly effective for this invention. The catalyst (or mixture of catalysts) is present in a catalytic amount, typically from about 0.01 to 1.0, preferably from 0.015 to 0.10 parts by weight per 100 parts by weight of resin, i.e. parts of resin per hundred.

Still other methods for cross-linking the present polyolefin mixtures can be used. For example, a combination of an electron beam and a crosslinking activator or multifunctional monomer such as ethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, diethylene glycol diacrylate, diallyl phthalate, triallyl cyanurate or pentaerythritol tetraacrylate can be used for successful crosslinking of the products according to this invention. "Radiation Processing of Polymers", published by Hanser Publishers, Munich, Vienna, New York and Barcelona, and edited by A. Singh and J. Silverman, provides further details regarding radiation crosslinking technology.

The above-mentioned methods for achieving cross-linking in polyethylene should be considered illustrative and not limiting. The process phase where cross-links are formed between different polymer molecular chains is also usually referred to as the "curing phase", and the process itself is also usually called "curing".

The present olefin mixtures may contain further additives, such as, for example, heat stabilizers, irradiation stabilizers such as UV stabilizers, pigments, colorants, extender materials, filler materials or processing aids. These are advantageously incorporated before the cross-linking step.

The mixtures according to this invention can be used in many different applications, and are particularly suitable for such applications where short curing times are either necessary or desirable. According to the present invention, a cross-linked polyolefin mixture obtained by curing the mixture according to the invention can be used for the production of an article selected from the group which includes films, extrusion-coated and laminated foils, extruded and calendered plates and foils, metal wire and cable coatings, sealing strips and seals, gaskets , articles made of foamed and/or sponge-like crosslinked polyolefin with either closed or open cell structure and/or combinations thereof, hollow containers, hollow tanks, medical instruments, molded cups, drapes and covers, healthcare and personal care articles, fibers, tapes, hoses, tubes and stockings, bellows, boots, gaiters, footwear, shoe soles and uppers, made from crosslinkable polyolefin blends. The mixtures according to this invention can be converted into finished articles or component parts intended for incorporation in the manufacture of other articles by means of well-known polymer processing methods such as metal wire and cable extrusion, rotational molding, profile extrusion, injection molding, compression molding, transfer molding, overforming extrusion (" overmolding extrusion"), blow molding, injection blow molding, thermoforming, drop molding, pressure blowing, slot die extrusion, plate die extrusion, foam extrusion, blown film extrusion, monoband and monofilament extrusion and powder coating.

Examples

The following examples illustrate certain specific embodiments of this invention. Unless otherwise stated, all parts and percentages are by weight.

A series of SLEPs were produced in a solution process mini-plant with two reactors, referred to herein as the "primary reactor" and the "secondary reactor", connected in series. The primary reactor was used for continuous polymerization of a feed mixture comprising an ethylene/1-octene/solvent mixture and hydrogen gas in the presence of a metallocene catalyst as described in US patents 5,272,236, 5,278,272 and 5,380,810. The partially reacted product from the primary reactor was fed continuously to the secondary reactor through the series connection between these reactors, where the product was caused to react with additional ethylene, again in the presence of the same metallocene catalyst, under reaction conditions that produced a polymer with a different Mn, generally a lower M„ than the Mn of the polymer formed under the primary reactor conditions. Such conditions generally included operation of the secondary reactor at a higher temperature than that of the primary reactor. Polymer mixtures corresponding to examples 1-4 were made by manipulating the reaction parameters for the primary and secondary reactor.

Example 1 is prepared, for example, by introducing a mixture comprising ethylene (C2), 1-octene (C8) and hydrogen gas (H2) into the primary reactor at the following rates:

SCMH = standard cubic meter per hour (that is, the rate in cubic meters at standard temperature and pressure).

The primary reactor was maintained at a temperature of 70°C, and a product stream was transferred from the primary reactor to the secondary reactor at a rate of 23.6 kg per minute. hour, together with 7.71 kg of ethylene per hour, while the secondary reactor was maintained at a temperature of 120°C forming a final product at a rate of 36.5 kg/hour.

Comparative Examples C-1 - C-14 are examples of comparative SLEPs, C-15 is an example of a homogeneous polyethylene polymer, C-16 - C-19 are examples of comparative LLDPE polymers, and C-20 and C-21 are examples of comparative VLDPE polymers.

Comparative Examples C-11 - C-20 are commercially available ethylene polymers. Engage® type polyolefin elastomers and Affinity® type polyolefin plastomers are both SLEP. Dowlex® and Attane® are LLDPE and VLDPE resins, respectively, manufactured and sold by The Dow Chemical Company. Tafmer™ is a homogeneous linear polyethylene interpolymer supplied by Mitsui (Japan).

Also, comparative examples DR-A - DR-M were made by drum mixing pairs of SLEPs and extruding the mixture. The extrudate was cooled and pelletised, and then extrusion of reactive silane was carried out with these pellets. The characterization of the two-component mixture samples is listed in Table III below.

Each polymer mixture gave a double peak chromatogram by GPC analysis before cross-linking. The results and their characterization are listed in Tables I-111 and XI below.

The data from Table IV show that the processability of a crosslinkable polyethylene blend is related to the total number average moment of its molecular weight M„. The shear rate at which the onset of surface melt fracture occurs as a function of total M„ for a series of substantially linear ethylene polymer blends is listed in Table IV. Fig. 5 illustrates the dependence, in terms of onset of surface melt fracture, on total M„ for the series of substantially linear ethylene polymer ("SLEP") blends shown in Table IV.

Table V shows the dependence, in terms of the melt viscosity measured at 220°C at 1800 sec<*1> for polymer according to the state of the art, and samples according to the invention, on total Mn. Fig. 6 illustrates this dependence and clearly shows that the melt viscosity essentially depends on total Mn.

Peroxide-cured examples

The mixtures listed in Table VI were compounded into crosslinkable preparations according to recipe 1 using the following method: 85% by weight of the polymer

was added in a 2 liter internal mixer of the type Farrel 89M5030 and mixed until the temperature of the material reached 80°C (about 2 minutes). A total batch weight of 1335 grams was used. The remaining ingredients were then added and mixed for a further period of time until the total cycle time from the start reached 5 minutes, after which the batch was withdrawn from the mixer. The outlet temperature was typically 100-110°C. The mixture was then milled on a Farrel 15 cm x 33 cm two roll mill for 2 minutes at 60°C using a friction ratio of 1.5:1 between rear and front roll speeds and a clamp of approx. 0.5 mm, before cutting off a 4 mm plate which was allowed to stand for 2 hours at 20°C before testing.

The time to optimum cure for the peroxide-crosslinkable preparations was determined at 160°C using an oscillating disc rheometer of the type

Zwick 4308 (1° rotor angle with a frequency of 100 min"<1>). The time to optimal cure, expressed as TB0, was calculated directly using the Zwick ODR computer program model 7049 3-2, version 06.07.89/07.07. 89 according to the method described in DIN 53529/T2.Table VI shows the dependence of the time before optimal hardening as determined by the oscillating disc rheometer using the method described above, as a function of total Mn for a series of peroxide-crosslinkable SLEP mixtures as described in recipe 1. The samples are identified by reference to the raw polymer from which the crosslinkable mixtures were made. Fig.

7 illustrates this dependence and clearly shows the higher than expected curing rate of the compositions according to the invention in relation to their total number average molecular weight. For example, example 2 is cured in 46.2 minutes at 160°C combined with an expected time of approx. 54 minutes, based on its number average molecular weight. Likewise, Example 3 was cured in 47.3 minutes at the same temperature compared to an expected curing time of no more than 60 minutes, and Example 1 was cured in 48 minutes compared to an expected curing time of 56 minutes at 160°C. The expected curing times were determined based on an extrapolation of the data for the prior art mixtures, i.e. based on an extrapolation of curing time combined with total Mn ratios of prior art mixtures.

Silane cross-linked examples

A series of polyethylene resins and predominantly linear ethylene polymers were reacted with a graft kit consisting of 1.519 wt% vinyltrimethoxysilane, 0.075 wt% dicumyl peroxide as a grafting initiator, and 0.025 wt% dibutyltin dilaurate as a crosslinking catalyst, based on the weight of polyethylene resin or predominantly ethylene polymer. For the preparation of the graft set, 10 cm<3> Dynasylan Silfin 12 consisting of 92.5% vinyltrimethoxysilane and 7.5% dicumyl peroxide was mixed with 6.67 cm<3> Dynasylan Silfin 21 consisting of 96.2% vinyltrimethoxysilane and 3.8% dibutyltin laurate (Dynasylan Silfin 12 and Dynasylan Silfin 21 are both commercial products supplied by Huls). The mixture was added to 985 grams of the polymer sample in a closed drum. The contents were drum mixed for 1 hour and then fed to a 16 mm single screw extruder with L/D 28/1, which was fitted with a screw with a compression ratio of 2.5:1, equipped with a "cavity transfer mixer" which fed a string nozzle. The equipment had been manufactured by the Extrusion Centre, Plasticisers Engineering Ltd. (UK). It is also possible to meter the pre-mixed silane/peroxide/catalyst mixture directly to the extruder funnel constriction, although this method was not used for the purposes of the investigation. Extruder revolutions per minute was such that the residence time was between 3 and 7 minutes, and the resin melting temperature was approx. 220°C. When using this method, all resins were grafted to an equal extent. The extruded strands were cut with an air knife using a stream of dry compressed air to avoid premature curing due to contact with moisture. Press-formed plaques of the extrudates were prepared by placing the dry granules in a mold of nominal dimensions 160 mm x 160 mm with a thickness of 2 mm, at a temperature of 180°C, the mold being heated to 190°C and pressurized to 1.5 MPa for 6 minutes, after which the pressure was increased to 15.0 MPa for 3 minutes, and then the temperature was allowed to increase to 20°C at a cooling rate of 15°C per minute. minute using a hydraulic press, type Platen Presse 200, manufactured by Collins. The formed plaques were then cured at 23°C in air containing 80% relative humidity, or by placing them in a thermostated water bath heated to 60°C.

The rate of cross-linking was monitored by periodically removing the plaque and by taking a dog-bone shaped sample for heat setting analysis. This analysis involved placing the dog bone with ASTM dimensions in an oven at 200°C, and weights equivalent to a tension of 20 N/cm<2> were attached to the sample. The resulting elongation of the sample is noted. As the hardening state of the sample increased, the measured elongation decreased. The rate of elongation-reduction is therefore a measure of the rate of hardening. The procedure is fully described in publication 811-2-1 of the International Electrotechnical Commission Standard, published 1986. The industry standard considers that a satisfactory state of cure has been achieved if the heat setting (elongation at specified temperature) of the sample does not exceed 175% after 15 minutes under a load of 20 N/cm<2>. To determine the setting time corresponding to this heat setting value of 175% after 15 minutes under a load of 20 N/cm<2> at 200°C, the heat setting is measured using a different dog bone test at each of a series of setting times, and the resulting heat setting compared with time is plotted on log diagram paper. With very short curing times, the heat setting value can be very high, and the sample can crack before 15 minutes have passed. In this case, the elongation under load was measured just before the specimen broke. A best-fit line was drawn through the data points and the intersection with the 175% heat set value, giving the target material cure time for evaluation purposes.

Table VII shows the time dependence of target cure for a series of prior art LLDPEs (including VLDPEs and ULDPEs) and comparative SLEPs as a function of number average molecular weight when cross binning in air containing 80% relative humidity at 23°C according to the method indicated above. Fig. 8 illustrates this dependence and clearly showed that the target cure time increased dramatically as Mn decreased for both LLDPE and SLEP. Similar curves can be obtained for other polyolefin classes.

A further series of silane crosslinkable mixtures was prepared according to the methods described above, but the difference was that the crosslinkable mixtures were crosslinked by allowing the formed plaques to lie in water held at 60°C. The series includes mixtures based on mainly linear ethylene polymers, mixtures of mainly linear ethylene polymers and ordinary homogeneous polyethylene.

Table VIII shows the dependence of the time before curing of the target material for this series of polymer blends as a function of Mn when silane is crosslinked using water at 60°C according to the method indicated above.

Although fig. 5 and fig. 6 clearly shows that the workability improved as Mn for polyethylene was reduced, fig. 8 and fig. 9 it is clear that the time until target curing for the silane-crosslinked mixtures according to the state of the art increased dramatically as the primary M„ for the polyethylene sample was reduced. Fig. 9 also clearly shows that the preparations proposed in EP 584 927 follow the same pattern as would be expected based on their Mn value.

Table VIII also shows the dependence of the curing time for the samples according to the invention according to this description as a function of number average molecular weight and consequently workability, compared with the curing time for corresponding preparations according to the state of the art. Fig. 9 shows the examples according to the invention as boxes highlighted by means of arrows. The scale of the "time to target material cure" axis is logarithmic to more clearly illustrate the surprising and significant improvement in cure rate that can be achieved without loss of machinability using this invention. For example, the mixture according to the invention based on example 1 was cured within 2.79 hours compared to 31.35 hours for comparative example DR-B, and 104.3 hours for comparative example C-14, with similar number average molecular weight; example 2 was cured in water at 60°C within 3.67 hours compared with a curing time of 9.82 hours under the same conditions for comparison mixture DR-C with similar number average molecular weight. Under the same conditions, example 3 was cured within 17.43 hours compared to an expected curing time of over 1000 hours, and example 4 was cured within 31.16 hours compared to an expected time again of over 1000 hours.

Mixture example 1 according to the invention, which was cured within 2.79 hours, can be compared with comparison mixtures DR-F and DR-E, which were cured within 3.67 and 2.40 hours, respectively. Example 1 had a polymer melt viscosity at 1800 sec"<1> at 220°C of 112 Pa.s compared to 163 Pa.s for DR-F and 185 Pa.s for DR-E. Similarly, the workability of mixture example 4 according to invention, which was cured within 31.16 hours, is compared to the workability of comparative mix DR-B, which was also cured within 31.35 hours. The melt viscosity of the polymer at 1800 sec"<1> at 220°C for Example 4 was 44 Pa. s compared to 72 Pa.s for DR-B and consequently significantly better for the mixture according to the invention.

Table IX shows a summary of Mn1/MnZ and A,/(A, + A2) for the comparison mixtures and the mixtures according to the invention. Fig. 11 shows the same data graphically and illustrates the combinations of the MJM^ and AJ{ A^ + A2 ) parameters which provide the advantages of the invention.

Although the invention has been described in some detail in the foregoing specific embodiments, it must be understood that these embodiments are for illustrative purposes only. Many variations and modifications can be made by a person skilled in the field without deviating from the principle and framework of this invention.

Claims (21)

1. Polyolefin mixture comprising two or more olefin polymer components, characterized in that it meets the following conditions: M,/Mr2 <>> 7.0, Mn2 > 3000 and 0.7 > ( AJ( A, + AJ) > 0.15, where A,, Aj, Mn, and Mn2 are derived from the molecular weight distribution of the mixture obtained by means of gel permeation chromatography (GPC) by deriving the relative response (RR) as a function of molecular weight (MW) fitting RR and MW to the following function, which is a weighted sum of two log-normal distribution functions: by applying a non-linear regression technique while obtaining values for A,, Aj, m, n2, a, and a2, where MW is the molecular weight value for GPC, RR is the relative response that for an individual RR/MW data set is where the normalized height [i] is the GPC yield for the corresponding MW(i); m and a,<2> represent the mean and standard deviation of the first log-normal distribution; H2 and o22 represent the mean and standard deviation of the second log-normal distribution; A, <+> A2 = 1 and 0 < A, < 1; Mn1<=> 10^<1> exp. (-0.5 (ln(10) x a,)2) and M„j = 10<*12> exp. (-0.5 (ln(10) x o2)<2>), and the polyolefin mixture has a total density of less than 0.907 g/cm<3>. 2. Mixture according to claim 1, characterized in that it has a total density of less than 0.900 g/cm<3.> 3. Mixture according to claim 1 or 2, characterized by Mn2 being greater than 4,000. 4. Mixture according to any one of claims 1-3, characterized in that A^tA, + Aj) is less than or equal to 0.65. 5. Mixture according to any one of claims 1-4, characterized in that A/tA, + Aj) is greater than or equal to 0.2. 6. Mixture according to any one of claims 1-5, characterized in that it comprises two ethylene polymer components. 7. Mixture according to claim 6, characterized in that the two ethylene polymer components are selected from mainly linear ethylene polymers. 8. Mixture according to claim 7, characterized in that the mainly linear ethylene polymers comprise ethylene and an α-olefin comonomer with from 4 to 10 carbon atoms. 9. Mixture according to claim 8, characterized in that each of the mainly linear ethylene polymers has a molecular weight distribution (IvL/MJ defined by the formula: where the melt flow ratio (l10/la) is greater than or equal to 5.63. 10. Mixture according to any one of the preceding claims, characterized in that it has a density in the range from 0.850 to 0.900 <g>/cm3. 11. Crosslinkable mixture according to any one of the preceding claims, characterized in that it further comprises a crosslinking agent, an activator, a promoter or an accelerator. 12. Mixture according to claim 11, characterized in that the cross-linking agent is an unsaturated silane grafted onto the mixture. 13. Mixture according to claim 12, characterized in that the unsaturated silane is represented by the formula where R' represents a hydrogen atom or a methyl group; x and y are 0 or 1, with the proviso that when x is 1, y is 1; n is an integer from 1 to 12; and each R independently represents a hydrolyzable organic group, or a lower alkyl group having from 1 to 6 carbon atoms, with the proviso that not more than one of the 3 R groups is alkyl. 14. Mixture according to claim 13, characterized in that n is an integer from 1 to 4, and each R independently represents a hydrolysable organic group which is an alkoxy group with from 1 to 12 carbon atoms, an aryloxy group, an araloxy group, an aliphatic acyloxy group with from 1 to 12 carbon atoms, oximo or substituted amino groups, or a lower alkyl group of from 1 to 6 carbon atoms, with the proviso that not more than one of the 3 R groups is alkyl. 15. Mixture according to claim 11, characterized in that Mn1/Mn2 is greater than 7.0; Mn2 is greater than 4000 and A^A, + A2) is greater than or equal to 0.2 and less than or equal to 0.65. 16. Use of a cross-linked polyolefin mixture obtained by curing the mixture according to any one of claims 11 to 15 for the manufacture of an article selected from the group comprising, films, extrusion coated and laminated foils, extruded and calendered sheets and foils, metal wire- and cable coverings, sealing strips and seals, gaskets, articles made of foamed and/or sponge-like cross-linked polyolefin with either closed or open cell structure and/or combinations thereof, hollow containers, hollow tanks, medical instruments, molded cups, drapes and covers, articles for health care and personal care, fibres, tapes, hoses, tubes and stockings, bellows, boots, gaiters, footwear, shoe soles and uppers. 17. Method for the production of a polyolefin mixture, characterized in that it comprises the following steps: a) production of a first olefin polymer and a second olefin polymer; b) mixing the first and second olefin polymer such that the first and second olefin polymer are mixed homogeneously, providing a polyolefin mixture satisfying the following conditions: where Mn1, M^, A, and A2 are obtained by the molecular weight distribution of the mixture obtained by means of gel permeation chromatography (GPC) by deriving the relative response (RR) as a function of the molecular weight (MW) while fitting RR and MW to the following function , which is a weighted sum of two log-normal distribution functions: using a non-linear regression technique to obtain values for A,, Aj, m, a, and a2, where MW is the molecular weight value for GPC, RR is the relative response that for an individual RR/MW data set is RR[i] = Normalized height [i]/(log (MW [i-1] - log (MW [i])) where the normalized height [i] is the GPC yield for the corresponding MW (i); u,, and a,2 represent the mean and standard deviation of the first log-normal distribution; m and g22 represent the mean and standard deviation of the second log-normal distribution; j\ + A2 = 1 andO<A, < 1; Mn1 = 10<M1> exp. (-0.5 (ln(10) x a,)<2>) and Mn2 = 10^ exp. (-0.5 (ln(10) x o2)2); and the polyolefin mixture has a total density of less than 0.907 g/cm<3>. 18. Method according to claim 17, characterized in that the first olefin polymer is an ethylene polymer which is produced in a first reactor, and the second olefin polymer is an ethylene polymer which is produced in a second reactor. 19. Method according to claim 18, characterized in that the first ethylene polymer produced in the first reactor is transferred to the second reactor, where the second ethylene polymer is produced in the presence of the first ethylene polymer. 20. Method according to claim 18 or 19, characterized in that it is carried out in slurry phase, solution phase or gas phase. 21. Method according to any one of claims 17-20, characterized in that it further comprises adding a cross-linking agent to the mixture.